Mass spectrometry has become a powerful technique for the determination of the structure of organic compounds, and has been applied to polypeptides (proteins) to ascertain the amino acid sequences of such polymers.
Electron Transfer Dissociation (ETD) is a gas-phase ion/ion oxidation-reduction reaction that utilizes an anionic species to transfer an electron to a multiply charged cation, i.e., a polyprotonated (polycationic) organic or biomolecular compound, usually a polypeptide, resulting in the dissociation of the compound into structurally informative product ions. These dissociation product ions can then be analyzed by any suitable mass spectrometric technique. This is particularly useful when the molecular species is a protein or peptide, as amino acid sequence information can be obtained thereby. In typical implementations of ETD, both the reagent anions and the precursor cations are confined in at least two dimensions within a radio frequency (RF) electrodynamic field.
In the most commonly employed techniques, reagent and precursor (polycationic) ions are simultaneously trapped by the electrodynamic fields within two-dimensional (2D-linear) or three-dimensional RF quadrupole ion trapping devices that also serve as mass analyzers. Generally, ETD reaction kinetics are pseudo first order, as the number density of the reagent ions within the overlapping clouds of trapped reagent and precursor ions is much larger than that of the precursor ions. Therefore the rate of conversion of precursor cations to product cations is approximately proportional to the initial concentrations (number density) of reagent anions (which are relatively stable throughout the reaction period). Utilizing low m/z (mass-to-charge ratio) reagents achieves reaction rates that are faster than those achieved with higher m/z reagents by allowing the ion trap to be operated such that the intensity of RF confinement fields (applied electrode voltage levels) during the reaction are greater, enabling creation of a higher density reagent anion cloud in the confining RF quadrupolar field, and therefore providing correspondingly higher reaction rates, whilst also allowing the retention of low m/z product ions following the ETD reaction by maintaining a sufficiently low m/z (mass-to-charge) cutoff (LMCO). This allows most of the possible C- and N-terminal product ions to be retained by the device. The faster reaction rates allow the mass spectrometer to generate ETD product ion spectra at a higher rate (which translates to shorter effective “scan” times), enabling mass spectrometric methods that can more thoroughly interrogate purified analytes introduced from a chromatographic column. See, for example, U.S. Pat. No. 7,534,622, by certain of the inventors herein.